This invention is in the field of data communications, and is more specifically directed to the shaping of broadband signals prior to transmission.
The prevalence of wireless telephones in modern society, as well as in developing countries, has exploded over recent years. Much of the increased deployment of this technology is due to recently implemented communications technologies that have enabled higher quality telephone reception, at lower costs. In addition, these technologies are now enabling a wider functionality for wireless telephones than simply voice communications. Text messaging has become quite popular, as has the use of wireless telephones to receive and send email messages. Combined phone/camera handsets are also now available, by way of which the user can transmit a digital photograph as part of a wireless telephone communication. In some markets, music can be downloaded to a wireless telephone for enjoyment during commuting. It is contemplated that Internet browsing will become a popular wireless telephone service before long, given the ongoing advances in this field.
An important one of these technologies is Code Division Multiple Access (CDMA), which is becoming the predominant technology for modem wireless telephony. This well-known approach is a type of spread spectrum communications, in which the baseband digital signal that is to be transmitted is “spread” over a wider bandwidth by the application of a spreading code to the signal. In effect, application of the spreading code converts the baseband signal into a higher frequency signal, with frequency components spread over a wider frequency band, but that communicates the same data content. According to CDMA, a base station can communicate with many handsets (“users”) within its range by using a different spreading code for each user, each selected from a set of orthogonal spreading codes. The user despreads the received transmission by applying the same spreading code as used in transmission. Because the spreading codes are orthogonal, the transmissions to the various users do not interfere with one another; indeed, baseband signals that are spread with one of the orthogonal spreading codes appear to be Gaussian noise when another one of the orthogonal codes attempts to decode it.
The so-called third generation (“3G”) wireless telephone services are contemplated to use a 3G version of CDMA communications that is referred to as wideband CDMA (“WCDMA”, also known as “UMTS”, and a variation of which is referred to as “CDMA2000”). In WCDMA, the baseband signal (or multiple signals spread by orthogonal codes) is spread over a 5 MHz communications channel (or 1.25 MHz in CDMA2000), achieving high data rate communications suitable for data, video, and other 3G services while maintaining excellent noise immunity. In conventional communications systems, multiple 5 MHz channels are simultaneously transmitted from each base station, further increasing the number of users that can be served.
FIG. 1 illustrates a conventional architecture for a wireless telephone base station, supporting many users within its service range. This conventional spread spectrum communication base station 200 receives signals (typically audio signals, but which may also include text or graphics information in the 3G context) to be transmitted from multiple channels. Typically, each channel's signal stream is generated by a vocoder or the like (not shown). Base station 200 includes N symbol converters 100-1 to 100-N that converts the received signals, which are in the form of digital bitstreams, to encoded symbols according to the desired modulation scheme, such as QPSK (Quadrature Phase Shift Keying). The output of symbol converters 100 typically includes both in-phase and quadrature phase components, corresponding to complex symbols. Each channel includes a corresponding mixer 110, which applies a spreading code defined by corresponding spreading code circuit 112, for example a Walsh code, to the symbol stream from symbol converter 100 for that channel. The spreading codes are orthogonal to one another so that the multiple channels can be combined into a single transmitted signal, while permitting the data for each channel to be recovered by the application of a matching despreading code, as known in the art. In this conventional base station 200, the spread channel signal is applied to mixer 114, which applies a cell-specific scrambling code, generated by scrambling code circuit 116, to the spread spectrum signal. The multiple spread and scrambled complex channel data are then combined by circuit 120 into a single output symbol stream for transmission. The typical steps in generating the transmitted signal include upsampling in the digital domain by conventional upsampling circuit 124 to convert the symbol rate to the desired frequency range, conventional filtering applied by filter 126, conversion to an analog signal by digital-to-analog converter (DAC) 128, mixing with the desired RF carrier 136 at RF mixer 134, and amplification by RF amplifier 130 and transmission from antenna 132 of base station 200, as shown in FIG. 1. Those receivers within the service range of base station 200 will thus receive the transmitted signal, and effect the conventional descrambling, decoding, and demodulation to recover the communication for their corresponding one of the N transmitted channels.
In this conventional base station 200, the combined signal produced by circuit 120 is a time domain signal that is effectively a sum, at each sample point, of the spread and scrambled in-phase and quadrature-phase symbols for the N channels. Especially considering that the spreading codes for the N channels are orthogonal to one another, the amplitudes of the N channels at each sample point in time are uncorrelated and independent of one another. As a result, it is statistically likely, especially over a large number of sample times, that the amplitudes of the N channels can align with one another to create an extremely high amplitude peak at a given sample time. This peak amplitude can be very high as compared with the average amplitude over the transmission time.
However, the dynamic range of RF amplifier 130 is necessarily limited, especially in conventional systems in which cost is a competitive factor. In order for RF amplifier 130 to faithfully transmit all sample points without undue distortion, it must be able to amplify these peak amplitudes without clipping. For a given dynamic range, therefore, the average output power may be forced to a relatively low level to permit distortion-free amplification of the peaks. A low average power affects the signal-to-noise ratio of the transmission, however, reducing communication quality. Accordingly, an important concern in spread spectrum communications systems is the peak-to-average ratio (PAR).
By way of further background, the PAR is exacerbated in those systems, such as wireless base stations, that transmit multiple channels over each of multiple frequency bands. These systems are commonly referred to in the art as multi-carrier communications systems. FIG. 2 illustrates such a multi-carrier system, in which multiple summing circuits 1201 through 120m are each combined into a single symbol stream for transmission. In this example, N channels of spread scrambled symbols are summed by each of m summing circuits 120. The summed signals occupy a certain bandwidth at baseband, for example within on the order of 5 MHz as known in the art. In this conventional approach, the m 5 MHz symbol basebands are each mixed with a carrier frequency f1 through fm, and then combined at summing circuit 137. The spectrum of the resulting combined signal from summing circuit 137 thus occupies m non-overlapping frequency bands, one corresponding to each of the m summing circuits 120. This resulting combined signal is thus also subject to providing large peak amplitudes at any given sample time, with the possibility of even a higher PAR considering that a larger number of independent channels (i.e., N times m combined channels) are involved.
It is known in the art to provide circuitry or functionality to reduce the peak amplitudes of combined spread spectrum signals. Referring back to FIG. 1, conventional base station 200 includes such peak reduction functionality, in the form of peak reduction unit 122, which reduces or eliminates signal peaks that will exceed a specified maximum signal peak power level.
U.S. Pat. No. 6,009,090 describes a simple conventional approach to peak or crest reduction. According to this approach, peak reduction unit 122 compares the amplitude at each sample point in the combined symbol stream to a threshold value. If the amplitude exceeds the threshold, peak reduction unit 122 simply truncates the symbol amplitude to a desired level. It has been observed, however, that this conventional approach, while simple, may not eliminate all peaks in the transmitted signal, due to the effects of downstream filter 126. In addition, this approach can also introduce distortion into the transmitted signal.
By way of further background, another approach to crest factor reduction, or PAR reduction, is described in U.S. Pat. No. 5,621,762. According to this conventional approach, the in-phase and quadrature-phase component amplitudes of the symbols to be transmitted within sequence are analyzed by a peak suppression algorithm, for example as implemented in peak reduction unit 122. The algorithm anticipates the effects of downstream filters and other factors, in analyzing these symbol amplitudes. If the algorithm determines that a transition from one symbol to the next will result in an excessively large peak at the transmitter output, the amplitudes of the in-phase and quadrature-phase components are scaled from their nominal values for that symbol to avoid generation of peaks in the output signal. This reduces the peak power demand on the amplifier, and permits the average output power to be increased for a given amplifier dynamic range.
According to a similar approach, an example of which is described in U.S. Pat. No. 6,449,302, conventional peak reduction unit 122 (also referred to as a crest factor reduction unit) predicts the output of filter 126 based on the incoming symbol stream. Peak reduction unit 122 then performs crest factor reduction processing on the symbol stream to reduce predicted signal peaks that will occur as a result of filter 126, using the known impulse response function of filter 126. Typically, this digital predistortion is accomplished by a non-linear distortion function that is built into the baseband digital processing prior to transmission. The non-linear predistortion function is applied to each of the individual carrier symbol streams, when used in a multi-carrier context, and corresponds to the distortion that will be exhibited by the RF amplifier, but of opposite polarity. As a result, the output signal presented by the RF amplifier is a cascade of the two non-linear distortion functions (i.e., the crest factor reduction plus the amplifier distortion), equating to a linear system. It has been stated that this approach permits the use of a simple class AB amplifier for transmission, reducing cost and improving system efficiency.
While conventional peak reduction techniques are often capable of reducing individual peaks in the spread spectrum signal, it has been observed, in connection with this invention, that the reduction of one peak may result in the creation of one or more other peaks at nearby sample points. Specifically, conventional peak reduction requires the modification of not only the peak sample point, but also the modification of neighboring sample points in order to remain within the signal-to-noise requirements of the system. The adjustment of neighboring sample points according to conventional peak reduction techniques can cause those neighboring sample points to themselves become peaks after peak reduction, even though their amplitudes did not originally qualify as peaks. In addition, it has been observed that many conventional peak reduction techniques are not effective for multi-carrier signals, as are now commonly used in WCDMA and CDMA2000 transmissions.